Part 22

Another principle, almost equally general in its applicability, is that of the dissipation of energy, for which we are indebted in the first instance to Sir William Thomson. All forms of energy may be converted into heat, and heat tends so to diffuse itself throughout all bodies as to bring them to one uniform temperature. This is its ultimate state of degradation, and from that state no methods with which we are acquainted can transform any portion of it. When energy is possessed by a system in consequence of the relative positions or motions of bodies which we can handle, and whose movements we may control, the whole of the energy may be employed in doing any work we please; in fact, it is all _available_ for our purpose, or its _availability_ may be said to be perfect. Energy in any other form is limited in its availability by the conditions under which we can place it. For example, the energy of chemical action in a battery may be used to produce a current, and this to drive a motor by which mechanical work is effected, but some of the energy must inevitably be degraded into the form of heat by the resistance of the battery and of the conductor, and this portion will be greater as the rate of doing work is increased. The ratio of the quantity of energy which can be employed for mechanical purposes with the means at our disposal, to the whole amount present, is called the _availability_ of the energy. All forms of energy may be wholly converted into heat, but only a fraction of any quantity of heat can be transformed into higher forms of energy, and this depends on the temperature of the source of heat and of the coldest body which can be employed as a condenser, being greater the greater the difference between the temperatures of the source and condenser, and the lower the temperature of the latter. In every operation which takes place in nature there is a degradation of energy, and though some portion of the energy may be raised in availability, another portion is lowered, so that on the whole the availability is diminished. Thus, in the case of the heat-engine, work can be obtained from heat only by allowing another portion of the heat to fall in temperature; and, as originally stated by Sir William Thomson, "it is impossible, by means of inanimate material agency, to obtain mechanical effect from any portion of matter by cooling it below the temperature of the coldest of the surrounding objects," and to leave the working substance in the same condition in which it was at the commencement of the operations. Accepting this principle, Professor James Thomson showed that increase of pressure must lower the freezing point of water, for otherwise it would be possible to construct an engine which, working by the expansion of water in freezing, would continue to do work by cooling a body below the temperature of any other body available, and he calculated the amount of pressure necessary to lower the freezing point through one degree. The conclusion was afterwards experimentally verified by Sir William Thomson, and served to explain all the phenomena of regelation. Thus, like the principle of the conservation of energy, the principle of the dissipation of energy serves as a guide in the search after truth. But there is this difference between the two principles--no one can conceive of any method by which to circumvent the conservation of energy; but Clerk Maxwell showed that the principle of dissipation of energy might be overridden by the exercise of intelligence on the part of any creature whose faculties were sufficiently delicate to deal with individual molecules. In the case of gases, the temperature depends on the average energy of motion of the individual particles, and heat consists simply of this motion; but in any mass of gas, whatever the average energy may be, some of the particles will be moving with very great, and some with very small, velocities. By imagining two portions of gas, originally at the same temperature, separated by a partition containing trap-doors which could be opened or closed without expenditure of energy, and supposing a "demon" placed in charge of each door, who would open the door whenever a particle was approaching very rapidly from one side, or very slowly from the other, but keep it shut under other circumstances, he showed that it would be possible to sort the particles, so that those in the one compartment should have a great velocity, and those in the other a small one. Hence, out of a mass of gas at uniform temperature, two portions might be obtained, one at a high temperature and the other at a low, and, by means of a heat-engine, work could be obtained until the two portions were again at equal temperatures, when the services of the "demons" might be again taken advantage of, and the operations repeated until all the heat was used up.

Any theory which is brought forward to explain a phenomenon, or any process which is proposed to effect any operation, must in the first instance submit to the test of the application of these two principles of conservation and dissipation of energy; and any proposal which fails to bear these tests may be at once rejected. The essential feature of the science of to-day is its quantitative character. We must, for instance, not only know that radiant energy comes to us from the sun, but we must learn how much energy is annually received by the earth in this way; and, in the next place, how much energy is radiated by the sun in all directions in the same time. When we have learned this, we want to know what is the source of this energy; and no theory of the sun which does not enable us to explain how this constant expenditure of energy is maintained can be accepted. Last century it was possible to believe, with Sir William Herschel, that the greater part of the sun's mass is comparatively cool, and that it is surrounded by only a thin sheet of flame. To-day such a theory would be rejected at once, simply because the thin shell of flame could not provide energy for the solar radiation for any considerable time. The contact theory of the galvanic cell, as originally enunciated, fell to the ground for a similar reason. The simple contact of dissimilar metals could afford no continuous supply of energy to sustain the current. Applied to the steam-engine, the doctrine of energy teaches us, not only that, corresponding to the combustion of a pound of coal, there is a definite quantity of work which is the mechanical equivalent of the heat generated, and is such that no engine of which we can conceive is capable of deriving from the combustion of the pound of coal a greater amount of work, but it teaches us that there is a further limitation fixed to the amount of work obtainable. This limitation depends upon the range of temperature at our command; and, when the range is known, we can express the amount of energy realizable by a perfect engine working through that range as a definite fraction of the whole energy corresponding to the heat of combustion of the fuel. Thus, if we find that a particular engine realizes only 15 per cent. of the energy of its fuel in work done, we must not suppose that mechanical improvements in the engine would enable us to realize any considerable portion of the other 85 per cent.; for it may be that a theoretically perfect engine, working with its boiler and condenser at the same temperatures as those of the engine considered, could only realize 25 per cent. of the energy of the fuel, reducing the margin for improvement from 85 to 10 per cent., as long as the range of temperature is unaltered. To improve the efficiency beyond this limit, the range of temperature must be increased, that is, generally, hotter steam must be used.

The principles of energy are thus guides, not only to the scientific theorist, but to the practical engineer, and they have been established only through careful measurement. The simple observation of phenomena, and of the conditions under which they occur, could never have led to the establishment of such principles; and, though the carrying out of experiments which do not involve measurements is of great value, it is the careful measurement, however simple, which affords the highest training to the mind and hand, and without which any course of instruction in experimental physics is of little value.

The Hindoos used to regard the earth as a vast dome carried on the backs of elephants. The elephants themselves, however, required support, and were represented as standing on the back of a gigantic tortoise. It does not, however, appear that any support was provided for the tortoise. In some respects this figure represents the apparently perpetual condition of scientific knowledge. Phenomena are investigated, and are shown to depend upon other actions which appear simpler or more fundamental than the phenomena at first observed. These, again, are found to obey laws which are of much wider application, or appear to be still more fundamental; but it may be that we are as far off as ever from discovering the great secret of the universe, the ultimate nature of all things.

INDEX.

A.

Abbott, Faraday's letters to, 241, 246.

Aberdeen University, Maxwell appointed professor in, 284; Young's report on, 203.

Absorption, Rumford's experiments on, 185; of sun's rays by cloth of different colours, 99.

Academy of Sciences, Franklin nominated Foreign Associate of, 111.

Adjustment of the eye, Young's paper on the, 200.

Æpinus's completion of Franklin's theory, 77.

Air, Boyle's conception of the constitution of, 19.

Air-pump, Boyle's experiments with, 19; constructed by Boyle, 27.

American Independence, Declaration of, 113.

American Philosophical Society, foundation of, 61.

Ampère's theory, Faraday's views on, 257.

Anchor-ring experiment, Faraday's, 260.

Arago's experiment, 264.

Argand lamp, efficiency of, 188.

Armstrong gun, principle of the, 180.

Atmospheric electricity, Faraday's experiments on, 254; obtained by a pointed rod, 84.

Autobiography of Franklin, 39.

Availability of energy, 326.

B.

Baily, Francis, repetition of the Cavendish experiment by, 146.

Beats in music, explanation of, 209.

Beggary in Bavaria banished by Rumford, 164.

Bernoulli's, Daniel, molecular theory of gases, 299.

Boston, blockade of, 110.

=Boyle=, Hon. Robert, birth, 8; conversion, 11; first air-pump, 17; conception of the constitution of the air, 19; experiments with the air-pump, 19, _et seq._; argument on the cause of a vacuum, 23; experiments establishing his law, 25; statement of his law, 29; observations on cold, 32, and on the expansion of water in freezing, 33; experiments on induced magnetism, 34; the province of experimental science, 37.

Boyle's law, 29.

Brocklesby, Dr., death of, 208.

Brougham's criticisms of Thomas Young, 218.

Bumper, electrical, 80.

C.

Camera obscura, invention of, 2.

Canada balsam, stresses in, 298.

Candle-flame, effect of, in discharging electricity, 75.

Capacity, electrical, 137; Franklin's experiments on, 81, 89; Cavendish's unit of, 138; Cavendish's measures of, 134, 138; of disc, measured by Cavendish, 134.

Capillarity, 228.

Cascade method of charging Leyden jars, 77.

=Cavendish=, Hon. Henry, F.R.S., birth and parentage, 126; social habits, 127; appointed member of the R.S. Committee on Lightning-Conductors, 131; elected Foreign Associate of the French Institute, 132; death, 133; proof of the law of inverse squares, 135; experiment with the spheres repeated by MacAlister, 137; experiments on the torpedo, 140; experiments on the resistance of conductors, 142; discovery of Ohm's law, 143; view of latent heat, 144; apparatus for determining the melting point of mercury, 145; the Cavendish experiment, 146.

Cavendish experiment, 146; Laboratory, 288; Manuscripts, 134; Maxwell's work on the Manuscripts, 293.

City Philosophical Society, joined by Faraday, 245; Faraday's lectures to, 251.

Cold, Boyle's observations on, 32.

Collinson, Peter, present of, to the Library Company, 72.

Colour-blindness, Maxwell's experiments on, 296.

Colour-box, Maxwell's, 297.

Colours, effect of, on absorption of sun's rays, 99, 186.

Colours of the spectrum mixed by Boyle, 31.

Colour-top, Maxwell's, 284, 295; Young's, 215.

Colour-vision, Maxwell's theory of, 294; Young's theory of, 214.

Commonplace-book, Faraday's, 253.

Compound-interest principle, 316.

Condenser, use of, in induction coils, 321.

Conduction of heat, Rumford's experiments on, 186.

Conductors, multiple, flow of electricity through, 141.

Conductors necessarily opaque, 307.

Conservation of energy, Maxwell's statement of the principle of, 325.

Copley Medal awarded to Franklin, 66, 74.

Cork, Earl of, autobiography of, 5.

Creeping of electricity on glass, 139.

Crystalline lens, fibrous structure of, 200; mode of adjustment of, 201.

Cuneus's discovery of the Leyden jar, 4.

D.

Davy, Sir Humphry, appointed professor at the Royal Institution, 174; letter of, to Faraday, 244.

Declaration of American Independence signed, 113.

Defence of the American Colonies against France and Spain, 62.

Degree of electrification, 137.

De la Rive's invitation to Faraday, 249.

Density of the earth, determinations of the mean, 146.

Desaguliers on electrics and non-electrics, 4.

Diagram of colour, Young's, 215; Maxwell's, 295.

Diamagnetism discovered by Faraday, 274.

Diamonds burned by Davy, 250.

Dichroism of _Lignum nephriticum_, 30.

Discharge, electrical, difference between positive and negative, 87.

Dissipation of energy, principle of, 326.

Distilled water, resistance of, 142.

Double refraction explained by Huyghens, 219.

Dufay showed that all bodies could be electrified, 4.

Dynamical nature of heat, suggested by Bacon, 2, 32; maintained by Boyle, 32; investigated by Rumford, 189; established by Joule, 193, 324.

Dynamical top, Maxwell's, 285.

Dynamo, constructed by Wheatstone, 318; action of, 319; essential feature of, 319.

E.

Effect of points in discharging electricity, 74.

Electrical picnic, 80.

Electrical Standards Committee, 287.

Electric intensity, 137; potential, 137.

Electricity, first obtained from clouds, 74; velocity of, 93.

Electrics and non-electrics, 3.

Electrolysis, Faraday's laws of, 266.

Electro-magnetic induction, discovered by Faraday, 259; Maxwell's statement of the laws of, 301.

Electro-magnetic theory of light, 306.

Electro-motors, 313.

Electro-tonic state, conceived by Faraday, 264; explained by Maxwell, 302.

Energy of Leyden jar resident in the glass, 79.

Eriometer, Young's, 223.

Ether, Maxwell's illustration of the possible constitution of, 302.

Expansion of water on freezing, 33.

Extra current, 268.

F.

=Faraday=, Michael, birth, 238; life in Jacob's Well Mews, 238; becomes an errand-boy, 239; apprenticeship, 239; attends lectures at Tatum's, 240; constructs a voltaic pile, 241; letters to Abbott, 241, 246; starts as a journeyman, 243; application to Davy, 243; appointed assistant at the Royal Institution, 245; joins the City Philosophical Society, 245; opinions respecting lectures, 246, 247; journey with Davy, 248; acquaintance with De la Rive, 249; crosses the Alps, 249; at the Academia del Cimento, 250; returns from the Continent, 251; lectures to the City Philosophical Society, 251; commonplace-book, 253; atmospheric electricity apparatus, 254; marriage, 255; discovery of electro-magnetic rotation, 255; of the earth's action on a current, 256; letter to E. de la Rive, 256; views on Ampère's theory, 257; elected F.R.S., 258; appointed director of the laboratory at the Royal Institution, 258; work on optical glass, 259; discovery of induced currents, 259; institutes Friday evening lectures, 259; anchor-ring experiment, 260; magneto-electric machine, 262; obtains induced current by action of the earth, 262; obtains "magnetic spark," 262; explanation of Arago's experiment, 264; laws of electrolysis, 266; proves the identity of frictional and voltaic electricity, 266; experiments on self-induction, 268; diagrams of lines of magnetic force, 269; conception of lines of electric force, 270; ice-pail experiment, 270; butterfly-net, 270; experiments on specific inductive capacity, 272; appointed scientific adviser to Trinity House, 273; appointed member of the Senate of the University of London, 273; discovery of the electro-magnetic rotation of the plane of polarization, 273; investigations in diamagnetism, 274; joins the Sandemanian Church, 275; lectures before the Prince Consort, 275; retirement to Hampton Court, 277; death, 277; lines of force investigated by Thomson and Maxwell, 300.

Forbes's, Principal, opinion of Young, 194.

Foucault's measurement of the velocity of light, 220.

_Fovea centralis_, insensibility of, to blue light, 298.

Franciscus Linus, funicular hypothesis of, 25.

=Franklin=, Benjamin, autobiography of, 39; birth, 40; on the disputatious temper, 42; method of learning prose composition, 43; tries vegetarianism, 44; adopts the Socratic method, 44; first voyage to England, 48; experience as a journeyman in London, 49; views on beer as a food, 49; marriage, 54; endeavours to attain moral perfection, 56; method of reconciling an enemy, 60; elected F.R.S., 66; second voyage to England, 70; begins electrical experiments, 72; electrical papers ridiculed by the Royal Society, 73; discovers the effect of points, 74; one-fluid theory of electricity, 76; theory of the Leyden jar, 78; invention of the lightning-rod, 83; golden fish, 85; view of the nature of light, 86; kite, 88; experiments on capacity, 81, 89; experiments on electrical induction, 90; proof of the absence of electricity in a hollow conductor, 91; third voyage to England, 102; examination before the Parliamentary Committee, 105; nominated Foreign Associate of the Academy of Sciences, 110; signs the Declaration of Independence, 113; sent to Paris, 113; made Minister Plenipotentiary to the Court of France, 116; signs the Treaty of Peace, 119; elected President of Pennsylvania, 120; death, 122.

Fresnel, awarded the Rumford Medal, 233.

Fresnel's repetition of Young's experiments, 225.

Friction as a source of heat, Rumford's experiments on, 189.

Friday evening lectures instituted by Faraday, 259.

G.

Galileo and Torricelli on the pressure of the air, 16.

Garnett, Dr. Thomas, professor at the Royal Institution, 173.

Gilbert, Dr., founder of electrical science, 3.

Göttingen, Young's university course at, 206.

Graham Bell's telephone, 319.

Gray, Stephen, discovers electric conduction, 3.

Grimaldi's fringes explained by Young, 222.

Gunpowder, Rumford's experiments on, 179.

H.

Halos, coloured, Young's explanation of, 224.

Hawksbee's observations on capillary attraction, 228.

Heat, a form of energy, 32; generated by friction in vacuum, 32; generated by friction, Rumford's experiments on, 189.

Herapath's explanation of gaseous diffusion, 299.

Herschel's, Sir John, comments on Young's principle of interference, 208.

Hicks's, Principal, investigations on the influence of temperature on gravitation, 184.

Hieroglyphics, Young's work on, 234.

Hobbes, opposition of, to Boyle, 25.

Hollow conductor, Franklin's experiments on, 91; Cavendish's experiments on, 135; Faraday's experiments on, 270.

Honorary degrees conferred on Franklin, 66, 101.

Hooke's law, 229.

Hooke, Theodore, founds the Royal Society, 14.

Huyghens's explanation of double refraction, 219; principle, 218.

Hydrogen, electro-chemical equivalent of, 267.

I.

Ice-pail experiment of Faraday, 270.

Identity of frictional and voltaic electricity, 266.

Induced currents, discovered by Faraday, 259; explained by structure of ether, 304; from earth's action, 262.

Induction coil, 320.

Induction, Franklin's experiments on, 90; self, 142, 306.

Induction machines, principle of, 316.

Insulators for lightning-rods, 96.

Interference, principle of, discovered by Young, 208; spectra of, obtained by Young, 225.

Invisible college, 13.

J.

Jenkin, William, discovery of the "extra current" by, 268.

Joule and Thomson's determination of the heat absorbed by air in expanding, 324.

Joule, Dr., establishment of mechanical theory of heat by, 193, 324.

Joule's law, 324; proof that heat and energy are equivalent, 324; determination of the mechanical equivalent of heat, 325.

Junto Club, formation of the, 51.

K.

Kelland's, Professor, edition of Young's lectures, 212.

Kinnersley commences lecturing, 73.

Kite, Franklin's, 88.

Knobs _versus_ points, 95.

L.

Laboulaye's comments on Franklin, 38.

Laplace's theory of Saturn's rings, 285.

Latent heat, Black's theory of, 144; Cavendish's views on, 144.

Leonardo da Vinci's observation of capillary attraction, 228.

Leyden jar, discovery of, 4; energy of, resident in the glass, 79.

Leyden jars charged by cascade, 77.

Light, Franklin's view of nature of, 86; Maxwell's electro-magnetic theory of, 306; rotation of the plane of polarization of, 273.

Lightning, effects of, on Newbury steeple, 92.

Lightning-protectors, Maxwell's, 294.

Lightning-rod, illustrations of the, 83.

_Lignum nephriticum_, dichroism of, 30.

Lines of force mathematically investigated by Thomson and Maxwell, 300.

Lines of magnetic force fixed by Faraday, 269.

Luminiferous ether, the vehicle of electrical action, 227; illustration of the possible constitution of, 302.

M.

Magdeburg hemispheres, experiments with, by Otto von Guericke, 17.

Magic squares, Franklin's proficiency in, 66.

"Magnetic spark" obtained by Faraday, 262.

Magnetization by induction, Boyle's experiments on, 34.

Magneto-electric machine, Faraday's, 262, 314.

Magneto-electric machines, Wilde's, 318; objects to be aimed at in the construction of, 315.